The present invention relates to the art of plastics materials and plastics processing, and more particularly relates to the art of making plastics composites, particularly from recycled materials.
Synthetic polymers such as polyolefins, particularly polyethylene, have come into ever increasing use for many household applications such as packages for consumer goods, wrapping film, bags and the like. Modern methods of making these polymers have created strong and economical materials which have gained progressively increasing acceptance by consumers. These materials are extraordinarily durable, and are essentially impervious to decay. Modern production methods make articles from these materials so economically that the same often are regarded as disposable items, to be used once and discarded. There has been a growing realization that the discarded materials should not be disposed of in landfills or by incineration, but rather should be reclaimed to recover these materials in usable form.
Scrap consisting of a single type of polymer ordinarily can be recycled by standard industrial processes. However, mixed plastic wastes pose more difficult problems. Ordinarily, plastics collected from consumers such as those retrieved from household wastes incorporate many different types of plastics and accordingly are not normally considered suitable for recycling by the conventional techniques used with pure plastics. Such mixed waste streams are referred to in the recycling art as "postconsumer" wastes. Techniques have recently been developed for reclaiming the most valuable and most readily identifiable constituents of such a mixed waste stream. With the development of improved techniques for recycling soft drink containers made principally of polyethylene terrepthalate ("PET") with or without polyethylene parts, it has become economically feasible to segregate such bottles from the mixed waste stream and recycle the so segregated bottles. These bottles constitute an appreciable fraction of the incoming waste stream and can be identified by their size and shape so that it is feasible to segregate the soft drink bottles manually. Likewise, milk containers, which are almost universally formed from high density polyethylene ("HDPE") constitute a substantial and readily identifiable fraction of the waste stream which can be segregated manually. Once the milk containers have been segregated from the remainder of the waste stream and cleaned, they provide essentially pure high density polyethylene which can be recycled in conventional ways.
These efforts, however, provide only a partial solution to the problem. After segregation of the PET and HDPE containers, there is still a fraction, commonly referred to as "tailings", which consists of many different types of plastic items, including films, containers, bags and the like. It is ordinarily not feasible to separate different items constituting this mixture as they are present in relatively small quantities and are difficult to identify. Moreover, some of the items in this mixture incorporate laminates or composites containing different types and/or grades of plastics. Ordinarily, the tailings fraction includes more than about 50 percent and typically more than about 75 percent by weight polyolefins, of which the majority ordinarily are polyethylene.
It has been observed heretofore that the tailings fraction can be comminuted, melted and melt-formed. However, the formed articles typically have relatively poor physical properties, such as low compressive modulus and low yield stress. These factors severely limit the types of products which can be made from the tailings fraction and hence limit the market for the recycled products. Likewise, the market for recycled high density polyethylene such as that derived from milk containers is limited by the relatively low strength and stiffness of this material.
There have accordingly been attempts heretofore to enhance the strength and stiffness of recycled HDPE. As set forth in the paper "Physical Characteristics and the Properties of Profile Extrusions Produced from Post Consumer Co-mingled Plastic Wastes", Renfree et al., 1989 Antec-47th Annual Technical Conference of the Society of Plastics Engineers, polystyrene can be coextruded with high density polyethylene recycled from milk containers. A coextruded blend including 50 percent polystyrene and 50 percent recycled HDPE (by weight) provides somewhat higher yield strength and compression modulus than other blends of plastics such as 100 percent tailings fraction. These results have not been widely accepted in light of the common knowledge in the art that polystyrene and polyethylene are immiscible and mutually incompatible plastics which ordinarily would be expected to yield poor properties if processed together. The methods according to this paper have been subject to unpredictable batch to batch variations in practice. Thus different batches of the polyethylene/polystyrene blend according to the Renfree et al. paper with ostensibly similar composition and processing conditions have produced markedly different physical properties. These inexplicable variations have posed a significant deterrent to any attempt at practical application of the Renfree et al. teachings regarding such blends. Moreover, the Renfree et al. paper does not say whether polystyrene can be used successfully as an additive with other polyolefin or principally polyolefin plastics such as the post-consumer tailings fraction mentioned above.
Accordingly, there have been significant unmet needs for still further improvement.